Use of weak links

On 2010/06/07 07:00 PM, Martin Gregorie wrote:
On Mon, 07 Jun 2010 08:42:16 -0700, Cats wrote:

On Jun 7, 3:41 pm, Martin
wrote:
On Mon, 07 Jun 2010 13:54:50 +0200, Andreas Maurer wrote:
On Mon, 07 Jun 2010 05:20:48 -0500, brian whatcott
wrote:

Brian wrote:
On Jun 6, 5:54 pm, John wrote:
Brian wrote:
True the 90deg AOA was justa theoritical way to look at the
loads on a stalled wing.
As soon as the wing stalls, the load decreases.

Yes it decreases, but there is some load on it still. Even a
stalled wings produces some lift just not nearly as much as a
flying wing.

Brian C.

How about drag?

Forget the drag - you cannot go fast enough with 90 degrees AoA to
produce any significant amount of drag.

There's another point that hasn't been mentioned - many winches can't
pull in cable at anything like Vwinch for most gliders, e.g. most
single seaters have a Vwinch of 65kts or higher but I'm told our
Supacat's maximum cable speed is 55 kts.

This doesn't mean that it can't break gliders: try this thought
experiment. Launch without a weak link, wait until the glider is at
60-70 degrees from the winch and ramp the winch up to full power to
cause a gross overspeed. This is the situation that will put maximum
load on the wings and it is the situation where the weak link is
designed to fail.

At least one of these things is very likely to happen: (1) the pilot
pulls the bung
(2) a back release, if the glider gets far enough overhead of the winch
(3) the hook gets pulled out of the glider
(4) the wings break
(5) the cable snaps

arranged in increasing severity and (my guess) decreasing probability
since I think that one of the less severe events will happen before the
more severe ones.
snip

Think I'd place cable snaps at #3, hook gets pulled out at #4 and wings
break at #5 on the grounds that we practise cable breaks, the hook
pulling out shouldn't affect *this* flight and wings breaking certainly
will.

I was being pessimistic and assuming a new 3/16" steel cable is being
used and made a guestimate that a new cable is stronger than a set of
used glider wings. I'd certainly expect the hook to pull out before the
spars snapped. I've taken my hook out for a thorough clean-up and have a
pretty good idea of what its bolted to!

I'm also assuming that a less severe event occurrence will reduce the
load and prevent the more severe events from happening. e.g. pulling the
hook out will prevent the spar or cable from breaking.


Interestingly we actually had some engineer members intrigued by this go
and measure the ultimate strength of the winch cable in use.

So - dyneema is another case entirely, and I have no data for
multistrand cable joined with ferrules - but 2 metric ton breaking
strain single strand wire broke reliably at ~900kg at the knot joining
it to the ring on the weak link.

Ergo it is pretty redundant putting a black Tost weak link on.

Not suggesting that no weak link is a good idea, just that the chances
of simple mechanical failure on the cable is higher than you think.
Especially after the wire gets abraded and work hardened.

If you work out the weight of the cable hanging from the hook - it will
seldom exceed 150Kg. Most of the force vector on the hook is from the
winch pulling. In the limiting case this is pulling straight down. At
this point the winch effectively becomes a sling load. I have witnessed
a ham fisted combination of winch driver and Blanik L13 pilot do this.
The vertical component was sufficient to momentarily lift one side of
the winch off the ground. Clear air under the wheel and both jacks.

The Blanik was no doubt way above Vwinch and pulling back heroically to
slow things down.

The resultant massive birdsnest came from the cable failing at the
attachment to the yoke - not from the blue weak link.

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